Specific oligonucleotide sequences are very useful tools in detecting complementary nucleotide sequences. The two requirements of the nucleic acid probe are a sequence specific signal and the formation of elements which will convert single hybridization events into multiple detectable events. In current enzymatic methods for preparing labeled probes, radioactive or biotinylated nucleotides are introduced into the probes by the use of polymerizing enzymes like DNA polymerase or terminal transferase. Methods are also available for introducing single enzymes or hapten molecules into DNA chemically, but these singly tagged probes do not generate enough signal, thus lacking the sensitivity needed for detecting complementary sequences in biological samples.
For example, Ward et al. in U.S. Pat. No. 4,711,955, discloses a procedure for derivatizing nucleotides with chemical determinants. The derivatized nucleotides are then enzymatically polymerized. Thus, these analogs function as substrates for nucleic acid polymerases. For this purpose, it is crucial that the chemical determinants not be placed on ring positions that sterically, or otherwise, interfere with normal Watson Crick hydrogen bonding potential of the bases.